What happened when Griffin injected mice with the pneumonia-causing strain of bacteria that had been heat-killed?

The mice stayed healthy

Explanation:

Griffith used two pneumococcus strains in which one strain was virulent strain and another strain was non-virulent strain. When Griffith injected non-virulent strain then mice remained healthy. Then Griffith injected virulent strain in mice which caused pneumonia in the mice and the mice died.

Then he injected heat-killed virulent strain in the mice and mice remained healthy after that he mixed heat-killed virulent strain with normal non-virulent strain and saw that the mice get killed. He concluded that the non-virulent strain took something from virulent strain which transformed non-virulent strain into virulent strain.

Therefore mice stayed alive and healthy when Griffith injected mice with pneumonia-causing strain which was heat-killed.